Signal communication devices, for example, microcomputers, printers and disc or tape drives often are connected in daisy chain, star or other architectures. Normally, each device has two ports and is connected to other devices using twisted pair wire. The devices communicate electronic signals to each other over the wires. The wires are terminated with resistors at the first and last device in the network. The resistors are selected according to the characteristic impedances of the wires and usually connect the signal lines to a source of power or to ground. Each signal line and its corresponding return path (source of potential or ground) form a transmission path, typically in a single twisted pair wire. The terminating impedance is coupled between the wires across the signal path. The resistors are usually either provided within each device or added externally at both ends of the chain. Two different approaches are commonly used.
The simplest approach is to plug a cable adaptor containing terminating resistors for each wire into the unused port of the end device. When the end device is removed from the chain or moved to a different place on the chain, then the cable adaptor must be removed from the moved device and connected to the unused port of the new end device.
The second approach has been to include a removable package of terminating resistors in a socket on the printed circuit (PC) board of the device. By removing an access panel on the device, the package can be reached and manually pulled off or pushed into the socket on the PC board depending upon the location of the device on the chain.
These two approaches have the drawback that the resistors are in a separate unit which must be manipulated and stored independently of the device. In addition, the cable adaptor is generally packaged, sold and shipped separately from the devices which greatly increases its cost. The removable resistor package requires the device user to open up an access panel on the device housing and locate the correct package on the PC board. Many users are reluctant to open up housings and remove packages from the PC board.
A third approach is to intercept all signals on the bus and then regenerate and retransmit them with appropriate impedances so that the terminators are coupled to the bus whenever the device is active and receiving signals. The StarLink products made by Western Digital, product numbers 8000SH, 8000SH2 and 80003SH, for example, use this approach. This avoids the separate package of resistors and frees the user from manipulating resistors by hand, but the circuitry required is complex and expensive.
In some daisy chain systems such as the Small Computer System Interface (SCSI) bus system currently used, for example, in products of Apple Computer Inc., each device is addressable for communication purposes, and each device is assigned an address. The address for each device is usually assigned by setting a rotary switch on the exterior of the device to the desired address, or by removing and installing jumpers on the PC board through an access panel. Devices in the chain can find out the address of their neighbors by using standardized interrogation protocols. In these systems installation is complicated further because the user, when configuring his system, must select addresses for the devices as well as install or remove the terminating resistors.